1. Field of the Invention
This invention relates to an optical device for providing uniform irradiation to a plane surface. The device is particularly useful in an apparatus for laser treatment of metals and semiconductors.
2. Description of the Prior Art
A broadly wavelength-tunable alexandrite laser was disclosed by Walling et al., in U.S. Pat. No. 4,272,733. The Patent suggested applications for the alexandrite laser such as welding, nuclear fusion, isotope separation, photochemistry, etc.
In general, lasers have been applied to many materials processing operations, including welding and soldering of metals, cutting and marking of both metals and nonmetals, and annealing of semiconductors. The latter application includes liquid and solid phase epitaxy, crystallinity growth, activation and diffusion of dopants, and elimination of lattice defects. The alternative to laser-annealing of semiconductors, furnace-annealing, requires heating and semiconductors at temperatures between 500.degree. and 1000.degree. for prolonged periods, which is neither as convenient nor as effective.
Laser-annealing of semiconductors, particularly silicon, has been disclosed in several technical publications as well as patents. Kaplan et al. in Electronics, Feb. 28, 1980, p. 137 summarize semiconductor annealer parameters and results achieved with various laser sources. Among the patents, U.S. Pat. Nos. 4,181,538 and 4,198,246 disclose novel methods for laser-treating silicon and include a great many references to early patents and publications on the subject.
The apparatuses used in laser-annealing of semiconductors have included both ruby and Nd:YAG lasers both pulsed and CW. Typically, the laser beam is focused onto the surface and scanned across the area to be treated. A simple laser-annealing apparatus was shown by Fan et al. in FIG. 1 of their U.S. Pat. No. 4,059,461. Although the shape and size of the spot on their sample is determined simply by a focusing lens, they make reference to alternatives such as "beam expanders, cylindrical lenses, mirrors, or other optical or mechanical elements."
Sharp, U.K. patent application GB No. 2,044,948 A, recognizing the disadvantage of non-uniform distribution of power in a laser beam used for heat treating a steel workpiece, disclosed a hollow beam shaper open at both ends and having internal reflective surfaces. A laser beam, after passing through the beam shaper, produces a rectangular area of substantially uniform power density on the surface of the workpiece.
Several laser beam shaping techniques were described by S. L. Ream in Laser Focus, Nov., 1979, p. 68. Among the optical devices he describes for use with CO.sub.2 lasers in hardening of steels are convex and concave beam integrators that consist of copper blocks having a large number of carefully positioned flat facets either turned in or attached to them. The beam integrator yields a flattened laser output profile, which corresponds to enhanced uniformity of laser intensity on the work surface. He includes semiconductor annealing among potential application areas for beam integrators.
Among the optical integrators described by Ream are the types disclosed by Dourte et al., in U.S. Pat. No. 4,195,913. They accomplish optical integration by the use of a plurality of reflective segments. These segments are positioned to divide a beam into a plurality of parts and reflect the various parts so that they intersect at a target. Among the configurations disclosed for the reflective segments is one in which the segments are positioned tangent to an imaginary cylindrical surface to produce a long narrow rectangular area of optical integration. Fabrication of the integrators disclosed by Dourte et al. is difficult and expensive, and beam uniformity is reduced by discontinuity and diffraction at the many segment edges.
Orthogonal cylindrical lenses for successive one-dimensional beam expansion were disclosed by Glenn, in U.S. Pat. No. 3,877,777.
Arrays of rectangular elementary mirrors for concentrating solar radiation were disclosed by Devin et al. in U.S. Pat. No. 4,148,564. Among the configurations they disclose is an array of rectangular plane mirrors arranged along a portion of a cylinder.